Plant Pathology AN INTRODUCTION TO PLANTS AND THEIR DISEASES FOR MASTER GARDENERS MAUREEN THIESSEN AREA COMMERCIAL ORNAMENTAL SPECIALIST
Plant PathologyAN INTRODUCTION TO
PLANTS AND THEIR DISEASES FOR MASTER GARDENERS
MAUREEN THIESSENAREA COMMERCIAL
ORNAMENTAL SPECIALIST
PathologyPathos – diseaselogos – study of
Plant pathology is the study of diseases affecting plants
What is disease?“The series of invisible and visible responses of plant cells and tissues to a pathogenic organism or environmental factor that result in adverse changes in the form, function or integrity of the plant and may lead to a partial impairment or death of plant parts or entire plant.”
-Dr. Datnoff, Plant Pathologist, LSU AgCenter
Disease triangleENVIRONMENT
PATHOGEN SUSCEPTIBLE HOST
Disease
Host
Envi
ronm
ent
Path
ogen
Diseases can be caused by abiotic and biotic factors…
ABIOTIC Non-living causes of disease – disorders
◦ Air/Water pollution◦ Nutrient or water imbalance◦ Temperature stress◦ Light stress◦ Chemical or mechanical injury
Can have effects on biotic disease factors
BIOTIC Diseases are caused by other living organisms
◦ Fungus◦ Bacteria◦ Virus◦ Nematodes◦ Parasitic plants◦ Algae and protozoa
Abiotic Factors
Sunscald Phosphorus deficiency
Drought Stress
Sulfur damage from atmosphere
Boron Toxicity
Abiotic Factors
Calcium deficiency-induced blossom end rot
Iron Deficiency
Magnesium Deficiency
2,4-D Damage
Winterkill on turfgrass
Biotic Factors
Signs vs. symptoms A sign is any visible part of the actual pathogen◦ Spores or fungal hyphae◦ Bacterial streaming◦ Parasitic plant parts◦ Nematode Eggs
A symptom is a manifestation of the damage caused by the pathogen◦ Chlorosis, discoloration◦ Wilting, stunting◦ Distortion, loss of vigor
How do we confirm biological pathogens?
Robert Koch – a German medical doctor and bacteriologist who discovered causal agents of anthrax, tuberculosis, and cholera.
Developed steps for identifying and confirming causal disease agents, known today as Koch’s postulates:
1. 1. The suspected causal agent must be present in every diseased organism
2. 2. The suspected causal agent must be isolated from the diseased host organism and grown in pure culture.
3. When a pure culture of the suspected causal agent is inoculated into a healthy susceptible host, the host must reproduce the specific disease.
4. The same causal agent must be recovered again from the experimentally inoculated and infected host, and have the same characteristics as the organism in step 2.
Koch’s Postulates for Dogwood Anthracnose
Basic Disease Cycle
Primaryinoculum
Secondary inoculum
Infection
Where do diseases come from? Disease cycle begin when pathogen inoculum contacts an infectable site on a susceptible host.◦ Can include fungal spores, bacterial cells, viruses, seeds◦ Carried by wind, water, insects, humans, equipment
Inoculum Types and Locations of Survival of Plant Pathogens Between Crops
Dissemination of Plant Pathogens
Attachment, Penetration, Infection•Fungal inoculum lands on plant surface, and adheres either by present moisture or excretion of enzymes and mucilaginous substances•Bacterial and viral cells can be washed in by water droplets or insects
Attachment, Penetration, Infection•Bacterial and viral cells can be washed in by water droplets or insects
•Nematodes enter directly or through root stoma
Effects of disease on vital processesInfection is the establishment and growth of pathogenic organisms and their parasitic effects in the host
As pathogens obtain food and resources for themselves, they interfere with vital physiological functions
Symptoms appear due to destruction of certain plant parts
Photosynthesis and Respiration Photosynthesis - process by which plants turn sunlight into energy
◦ Ability to create energy through photosynthesis is limited by:◦ Leaf necrosis◦ Spotting◦ Defoliation◦ Chlorosis (loss of green pigment)◦ Destruction of enzymes involved in photosynthesis.
Respiration – process by which living organisms burn energy to perform physiological processes.◦ Rate is increase in diseased plants, reserves depleted more quickly as plants
mobilize defense mechanisms
Photo: Alan Windham
Effects on Translocation•Translocation is the movement of water and nutrients through the plant vascular system.•Pathogens interfere with translocation by:
Rotting roots and stems, inhibiting their ability to absorb and transport water and nutrients
Producing cankers, galls, and root knots that block transport
Effects on Translocation
Clogging vascular tissue with growth (ex. Fusarium, Verticillium wilts)
Damaging protective cuticle and integrity of cells, thereby increasing water loss
Effects on Growth and Development•Several viruses and some fungal pathogens affect plants by changing genetic material• Curling, mottling, stunting
•Some pathogens destroy reproductive structures and seeds
Plant Defenses Plants encounter thousands of pathogens every day Infection fails to take place for most encounters due to:
◦ Lack of compatibility◦ Structural Barriers◦ Formation of toxins and other chemical defenses
This is what causes plants to be resistant
Structural Defenses Many defenses keep pathogens from landing or sticking to plant tissue Preexisting
◦ Waxy cuticle – water film resistant◦ Thick cell walls◦ Leaf hairs◦ Thorns
Structural Defenses Induced
◦ Plant cell receptors that recognize certain pathogens or their compounds can cause formation of defense structures to limit infection
◦ Cytoplasm changes◦ Cell wall thickening and
surrounding◦ Formation of corky barriers or
abscission layers◦ Tyloses
Chemical Defenses•Plants produce substances that kill or repel pathogens or neutralize pathogen toxins
•Phenolics and acids – “fungitoxic” and/or prevent spore germination
•Phytoanticipins (antimicrobials)
•Inhibit digestive pathogen enzymes
•Tomatine and avenacin in tomatoes and oats have membraneolytic hydrolytic activity – break down pathogen
•Can also be induced – plants have receptors for some pathogens that can elicit cascade of responses to wall off or kill infection
Fungal Symptoms
Types of Disease PathogensFungal
Bacterial
Viral
Parasitic plants
Nematodes
Fungal Diseases•Kingdom Fungi – Eukaryotic (nucleus containing) organisms that are heterotrophic, absorbing carbon fixed by other organisms (unlike plants)•Once considered to be plants without chlorophyll•Made up of hyphae – strands/branches that make up the body of the fungus – mycelium•Some are easily seen by their fruiting bodies, others remain hidden in the soil, some are unicellular (yeast)
Fungal Diseases•Nutrient uptake – Fungi secrete enzymes that digest material to be absorbed by the fungal hyphae.•Cause disease by maceration, cell destruction, and disruption of cell and whole-plant processes•Reproduce using spores housed in many different types of fruiting bodies•Fungal diseases spread by spores or transferal of hyphae
Some fungal life cycle examples P 437 vert basic rhizopus
447 plum pocket – diff kinds of spores may be involved
P 462 septoria leaf spot like on tomato
Rose powdery mildew p 451
Stem canker p 481 and pictures 477
Apple scap p 506 may be good replacement of one of the above
Pp 606 wood rotting mushroom fungi
Add your own pics of ganoderma
Apple ScabVenturia inaequalis
Powdery Mildew of Rose Sphaerotheca pannosa f. sp. rosae
Stem Cankers
Wood- Rotting fungiex. Gannoderma
Fungal-like organisms Different kingdoms, but grow, reproduce, and cause disease much like Kingdom Fungus
May have differences in cell wall structure, but still produce spores
Myxo and Plasmoidium are more like masses of cytoplasm, not def cell wall.
Oomycetes more like tru fungi ◦ Phytophthora, pythium, downy
Control of Fungal Diseases Plant diseases are nearly impossible to “cure”
Control is mostly prevention-based
Holistic Approach - Follow best management practices
Use techniques that preserve plant health and minimize inoculation◦ Resistant varieties◦ Only accepting disease free plant material and seeds◦ Minimize moisture , increase root aeration◦ Destroy diseased material◦ Sanitation of tools and environment
Chemicals (synthetic and natural) can also be used, but should be in combination with above methods, and used preventatively as well
If have time, chemical control How some synthetics (chlorothalonil, etc. work_
How neem oil and other oils work specifically for fungi
Consult ch 9
Bacterial diseases•Prokaryotic single-celled organisms – do not form structures
•Are transported through the vascular tissue and intercellularly
•Cause disease by multiplying and secreting toxins or clogging vascular tissues, maceration, cell destruction, toxins, and disruption of cell and whole-plant processes
•Can also have DNA-altering activity
Infection•Bacterial cells can enter through wounds, stomata, hydathodes
•Can be vectored by insects and nematodes as well•Cause spots, soft rots, wilts, cankers, galls,
Vascular Wilts◦ Bacterial cells colonize vascular cells, destroying their integrity and limiting
water and nutrient movement
Affected xylem
Affected phloem
Stem Cankers•Bacteria often enter through nodes or buds•Colonize and damage tissues•Causes girdling
Fireblight on PearErwinia amylovora
Galls◦Bacteria parasitize the genetic and metabolic machinery of host
◦Bacterial cells enter through wounds and introduce own genetic material to host cell through a Ti plasmid.
◦These newly integrated genes cause host cells to synthesize materials useful only to bacteria, and to divide rapidly, forming galls, or tumors.
◦Similar to the establishment of cancer
Crown GallAgrobacterium
Control Even more difficult to control than fungal diseases Many disease-causing bacteria survive in the soil - source of inoculum
◦ Sterilize media and soil, clean up and destroy diseased crops
Holistic approach Resistant varieties Practice good sanitation:
◦ Frequently clean or change tools, especially in grafting and pruning◦ Minimize production of wounds
Copper compounds and antibiotics have had limited success
Viral Diseases•Viruses are considered non-living, obligate parasites
•Much smaller than bacteria and fungi
•Consist of a nucleic acid (genome) and protein covering, sometimes its own enzymes
•Parasitizes the genetic and metabolic machinery of its host
•Cause disease by disruption of normal cell functioning• Up- or down- regulate production of normal enzymes, compounds, and hormones, thus
altering growth and appearance
•Symptoms most often will consist of abnormal growth – mottling, mosaic discolorations, leaf stem and root malformations, stunting, streaking, distortion, etc.
Viral Symptoms
Infection and Spread•Viral particles enter through tiny wounds in cell walls. Nucleic acids are copied by viral or plant enzymes
•Viral nucleic acid also codes for new viral “parts” – new protein coats, enzymes, and copies of genome. New viral particles are made
•New particles are spread between cells (they are tiny enough) and through vascular tissues
Infection and Spread Can be spread by humans, animals, insects, fungi, mites, nematodes. Can remain in and spread through pollen and seed. Put figure 14-17 here
Eriophyid Mites and Rose Rosette
Thrips and Tomato Spotted Wilt Virus
Control of Viruses•Hardest to control•Resistant varieties•Limit insects and other disease-spreading organisms•Transgenic plants (TYLCV, tomato mottle begomovirus)•Virus free culture of apical meristem tips•No viricides exist
Other Plant PathogensParasitic Plants
Examples include Dodder and Witchweed◦ Produce flowers and seed but little to no
chlorophyll◦ Seeds exist in soil and germinate much like
plants◦ Strands wrap around stems and send fungus-
like roots into plant tissues to harvest nutrients
◦ Usually controlled with herbicide
Nematodes Microscopic worms living freely in the soil
Feed on roots with piercing stylet, inject saliva with enzymes, suck nutrients
Enzymes in saliva cause degradation of plant tissues
Causes root knots and lesions, root destruction and therefore typical ill-health symptoms above ground
Root Knot Nematode (Meloidogyne)
•Cause extensive damage to many plant types (tomato, potato, carrot, peanut, dogwood•Causes galls, root stunting, and root necrosis•Control by rotation with resistant varieties, soil solarization or fumigation, nematicides.
Diagnosing1. What is the host plant?
2. How many of the host plants are affected?
3. Is the pattern of damage random or uniform?
4. What plant parts are affected?
5. What are the signs and symptoms?
6. What recent activity has happened near the plants? Weather, temps, chemicals…
7. Cultural history (irrigation, fertilization). Has the soil been excessively wet?
8. Are there any wounds or mechanical damage?
Resources Extension
◦ County Agent◦ Soil Plant, Pest Center◦ search.extension.org
Resources Plant Pathology by George Agrios. 5th Edition, 2005
What’s Wrong With My Plant? And How Do I Fix It? by David Deardorff and Kathyrn Wadsworth. 2009